rear angle of the wagon, while Tom
and Ralph were getting the posts for the outer ends of the fort.

When all was ready, the signal was given, and the various divisions
sprang to their allotted work. They marveled at the celerity with which
the fort was put up.

"I see a serious defect in the arrangement at one point," said Tom.

"What is that?" was the question from all.

"You see we have the fort ready, but it is adapted for one side of the
wagon only. It may be most important to have it arranged so that either
side of the wagon can be used for the fort."

"A fine suggestion," answered John. "That change can be made by having
both sides of the body so they can be let down."

George also had a good suggestion to make. "According to the
calculations we have fourteen cubic feet of material, and it is light
wood, at that. Why couldn't the fort be utilized as a raft, so as to
save the cutting of green timber, which is so heavy?"

"Well," said the Professor, smiling, "we are developing this at an
immense rate. The new idea is the starting point for an invention to so
arrange the sections as to make them act as floats. Here is a great
opportunity for the genius."

Tom and Harry conferred on this subject for some time, and after
luncheon, announced the plan: "It is fortunate that the axles of our
wheels project. It will be an easy matter to take the sections from the
sides of the body and attach one edge of each section to the projecting
ends of the axles, and then the two posts can be used as braces to run
up from the outer edges on the sections to the upper edges of the body.
The third section can be left under the axles where it now is."

John was delighted at the simple solution of this problem.

"I really believe," said Tom, "that we can adjust the sections in that
way while the wagon is moving, as it can be attached without any
difficulty."

It is wonderful how one improvement marks the advance stride for the
next. Invention is really nothing but a step by step movement; a little
addition here, another accretion there, and so on, so that invention has
been shown to be, not a matter of quantity, but of quality. The mere
bending of a wire, if it produces a new and useful result, is just as
much entitled to the dignity of an invention, as a room full of
intricate mechanism.

CHAPTER XVIII

TRAILING A WARRING PARTY OF NATIVES

"Professor, won't you tell us what the difference is between weight and